Filter, communication module, and communication apparatus

ABSTRACT

A filter includes a substrate; a signal line formed on the substrate and including an input terminal and an output terminal at either end of the signal line; and a first pair of resonant lines connected between the signal line and a ground portion, wherein the first pair of resonant lines are connected to the signal line at the same point.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 12/632,098filed on Dec. 7, 2009, which is based upon and claims the benefit ofpriority of the prior Japanese Patent Application No. 2008-329360, filedon Dec. 25, 2008, the entire contents of which are incorporated hereinby reference.

FIELD

The embodiment discussed herein is related to a filter which allows apredetermined frequency band signal to pass through.

BACKGROUND

In recent years, as well as the market of mobile communication equipmentsuch as a portable telephone growing, the service has becomeincreasingly sophisticated. Along with this, the frequency band utilizedby the communication network has shifted to a high frequency band of 1GHz or higher, and also, there is a trend toward a multiple number ofchannels.

FIG. 1 is a circuit diagram illustrating a configuration of a relatedhigh frequency variable filter. The high frequency variable filterillustrated in FIG. 1 includes a plurality of channel filters 101 a to101 c, and switches 102 a and 102 b. The passbands of the channelfilters 101 a to 101 c differ from one another. A high frequency signalinput from an input terminal 103 is output from an output terminal 104via one channel filter selected by the switches 102 a and 102 b. Byswitching the switches 102 a and 102 b, it is possible to change thepassband of the high frequency variable filter.

For example, Japanese Unexamined Patent Publications JP-A 10-335903 andJP-A 2007-174438 disclose the heretofore described kind of highfrequency variable filter including the plurality of channel filters andthe switches.

However, the configuration illustrated in FIG. 1 includes a number offilters equivalent to the number of channels. For this reason, as wellas the size of the high frequency variable filter increasing, a costalso increases. Also, a loss of signal occurs in each switch.

In recent years, attention has been drawn to a small variable filterusing an MEMS (Micro Electro Mechanical Systems) switch and an variablecapacitor. An MEMS device such as an MEMS switch may be applied to ahigh frequency band variable filter with a high Q (quality factor).

“D. Peroulis et al, “Tunable Lumped Components with Applications toReconfigurable MEMS Filters”, 2001 IEEE MTT-S Digest, p341-344”, “E.Fourn et al, “MEMS Switchable Interdigital Coplanar Filter”, IEEE Trans.Microwave Theory Tech., vol. 51, NO. 1, p320-324, January 2003”, and “A.A. Tamijani et al, “Miniature and Tunable Filters Using MEMSCapacitors”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 7,p1878-1885, July 2003” disclose the heretofore described kind of MEMSdevice.

The MEMS device, because of its small size and low loss, is often usedin a CPW distributed constant resonator (CPW: Coplanar Waveguide).

“A. A. Tamijani et al, “Miniature and Tunable Filters Using MEMSCapacitors”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 7,p1878-1885, July 2003” discloses a filter with a structure in which aplurality of MEMS variable capacitors straddle three distributedconstant lines. In this filter, by the variable capacitors beingdisplaced to change a gap between the variable capacitors anddistributed constant lines, it is possible to change the capacitance. Bychanging the capacitance of the capacitors, it is possible to change thepassband of the filter.

In “A. A. Tamijani et al, “Miniature and Tunable Filters Using MEMSCapacitors”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 7,p1878-1885, July 2003”, quartz and glass are used as substratematerials. Also, the drive electrodes of the variable capacitors aredisposed in a gap between a ground line and signal line formed on asubstrate. Also, the length of the lines is defined by the permittivityof the substrate.

In the heretofore known distributed constant filter, the lower thefrequency band, the larger the size. For example, the usable frequencyband of principal mobile communication equipment such as a portabletelephone is approximately 800 MHz to 6 GHz. However, when the frequencyband is 800 MHz to 6 GHz, as the wavelength is long, the size of thedistributed constant filter is too large for practical use. For example,in the event that a transmission line with an electrical length of λ/2is fabricated to be a 75Ω microstrip line working at 800 MHz by using aceramic substrate (permittivity ∈=9.4), the physical length beingapproximately 77 mm, it is difficult to put the filter into compacthandheld wireless communication usage.

By using a high dielectric substrate, it is possible to shorten thelength of the lines to some extent. However, when the substratepermittivity becomes higher, it not being possible to form a distributedconstant line with a high characteristic impedance, there will be nodegree of freedom in a filter configuration. For example, in the eventthat a microstrip line is formed using a substrate whose permittivity ∈is 80, even though a distance between the signal line and ground isincreased to 600 μm, a 50Ω (or other similar resistance) signal line mayonly take up a width of 20 μm. For this reason, a transmission lossincreases. Consequently, there is a limit to reducing the filter size byincreasing the substrate permittivity.

SUMMARY

A filter includes a substrate, a signal line formed on the substrate,including an input terminal and an output terminal at either end of thesignal line, and a first pair of resonant lines connected between thesignal line and a ground portion, wherein the first pair of resonantlines are connected to the signal line at the same point.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram of a configuration of a related filter;

FIG. 2 is a circuit diagram of a filter in the embodiment;

FIG. 3 is a circuit diagram of a configuration of a filter in which acoupling circuit is connected;

FIGS. 4A to 4N are circuit diagrams of the coupling circuit;

FIG. 5 is a circuit diagram illustrating another configuration of thefilter;

FIG. 6A is a plan view of a configuration wherein resonant lines areconnected to a common ground;

FIG. 6B is a plan view of a configuration wherein resonant lines areformed into an arc shape;

FIG. 7A is a plan view of a resonant line of a variable filter elementtaken along the line A-A in FIG. 6B;

FIG. 7B is a sectional view taken along Z-Z in FIG. 7A;

FIGS. 8A to 8G are sectional views illustrating a process ofmanufacturing the variable filter element;

FIG. 9 is a block diagram of a configuration of a communication module;

FIG. 10 is a block diagram of a configuration of a communication moduleincluding variable filters;

FIG. 11 is a block diagram of a configuration of a communicationapparatus; and

FIG. 12 is a block diagram of a configuration of a communicationapparatus including variable filters.

DESCRIPTION OF EMBODIMENTS 1. Configuration of Filter

1-1. Filter Including Pair of Resonant Lines

FIG. 2 is a circuit diagram illustrating a basic configuration of abandpass filter which is one example of a filter of this embodiment. Asillustrated in FIG. 2, with the bandpass filter of the embodiment, aninput line 2 a is connected to an input terminal 1. The input line 2 ais connected to a contact point 3. A resonant line 2 b and resonant line2 c are connected between the contact point 3 and ground. That is, theresonant lines 2 b and 2 c are connected in parallel, at the samecontact point, to a signal line connecting the input terminal 1 and anoutput terminal 4. An output line 2 d is connected between the contactpoint 3 and output terminal 4. Also, the input line 2 a, resonant lines2 b and 2 c, and output line 2 d are formed by a distributed constanttransmission line. In the embodiment, two resonant lines connected inparallel at the same point are defined as a “pair of resonant lines”.

The input-output impedance of the filter illustrated in FIG. 2 is takento be, for example, 50Ω. Also, the impedance of the resonant lines 2 band 2 c, being lower than at least the impedance of the input line 2 aand output line 2 d, is taken to be, in the embodiment, 20Ω as anexample.

The resonant lines 2 b and 2 c having a length of (λ/8)×n (n is apositive integer), in the filter illustrated in FIG. 2, they have alength of λ/4 (that is, n=2). λ is a wavelength (a resonant wavelength),in the distributed constant transmission line, of the frequency (theresonant frequency of the resonant lines 2 b and 2 c) of a signalextracted in the filter of the embodiment. Also, each of the resonantlines 2 b and 2 c means a resonator of which one end is connected toground, and which is formed by the distributed constant transmissionline. By connecting one end of each of the resonant lines 2 b and 2 c toground in this way, signals input into the resonant lines 2 b and 2 cvia the input line 2 a are filtered by being totally reflected from theground ends of the resonant lines 2 b and 2 c, enabling a desiredfrequency signal to be extracted from the output terminal 4.

A description has been given of an example in which the resonant lines 2b and 2 c are connected to ground, but it is also acceptable that theyhave an open end.

By connecting the resonant lines in parallel to the signal line, andconnecting the plurality of resonant lines in the same position in thesignal line, as heretofore described, it is possible to shorten a linelength between the input line 2 a and output line 2 d in comparison witha filter in which a plurality of resonators having a line length of λ/2are connected in series, as in a heretofore known technology, so it ispossible to reduce a size of the filter in a signal line direction.

Also, with the filter illustrated in FIG. 2, the pair of resonant lines2 b and 2 c are disposed in positions facing each other across thesignal line connecting the input terminal 1 and output terminal 4.Because of this, it is possible to dispose the resonant lines in highdensity, so it is possible to further reduce the size of the filter inthe signal line direction. Even in the event that the resonant lines aredisposed on the same side (one side) with respect to the signal line, itis possible to reduce the size of the filter in the signal linedirection.

1-2. Filter Including Plurality of Pairs of Resonant Lines

FIG. 3 is a circuit diagram of a filter including a plurality of pairsof resonant lines. With the filter illustrated in FIG. 3, an input line12 a is connected to an input terminal 1. The input line 12 a isconnected to a contact point 13 a. A resonant line 12 b and resonantline 12 c are connected between the connect point 13 a and ground. Thatis, the resonant lines 12 b and 12 c are connected in parallel to thecontact point 13 a. Also, the resonant lines 12 b and 12 c are connectedto the same position (the contact point 13 a) in the signal line.Meanwhile, on the output side of the filter, an output line 12 g isconnected between a contact point 13 b and output terminal 4. A resonantline 12 e and resonant line 12 f are connected between the contact point13 b and ground. That is, the resonant lines 12 e and 12 f are connectedin parallel to the contact point 13 b. Also, the resonant lines 12 e and12 f are connected to the same position (the contact point 13 b) in thesignal line. In the embodiment, a description has been given of anexample in which the resonant lines 12 b, 12 c, 12 e, and 12 f areconnected to ground, but it is also acceptable that they have an openend.

A coupling circuit 14 is connected between the contact point 13 a andcontact point 13 b. The coupling circuit 14 is a circuit which couplesthe contact point 13 a and contact point 13 b in the filter. Thecoupling circuit 14 may be realized by, for example, a capacitorconnected in series between the contact point 13 a and contact point 13b.

Also, in the filter illustrated in FIG. 3, it is preferable that theimpedance of the resonant lines 12 b, 12 c, 12 e, and 12 f is lower thanat least the input-output impedance of the filter. This is forconfiguring in such a way that signals (currents) input from the inputterminal 1 flow into the resonant lines 12 b, 12 c, 12 e, and 12 f. Inthe embodiment, as one example, the input-output impedance of the filteris taken to be 50Ω, and the impedance of the resonant lines 12 b, 12 c,12 e, and 12 f to be 20 Ω.

In the filter illustrated in FIG. 3, the signals input into the inputterminal 1 are input into the resonant lines 12 b and 12 c via the inputline 12 a. As one end of each of the resonant lines 12 b and 12 c isconnected to ground, a signal which meets a resonant condition of theresonant lines 12 b and 12 c is totally reflected from the ground endsof the resonant lines 12 b and 12 c, but a signal which does not meetthe resonant condition, by being grounded, or reflected to the input endside, is attenuated (a filtering). The signal totally reflected from theground ends of the resonant lines 12 b and 12 c is input into theresonant lines 12 e and 12 f via the coupling circuit 14. The filteringof the signal is carried out in the resonant lines 12 e and 12 f, in thesame way as heretofore described, and the filtered signal is output fromthe output terminal 4 via the output line 12 g. As only a predeterminedfrequency band signal is filtered in each resonant line, it is possibleto output the predetermined frequency band signal from the outputterminal 4.

Hereafter, a description will be given of a specific configuration ofthe coupling circuit 14. The coupling circuit 14 may be realized by onlythe capacitor, as heretofore described, but various other forms are alsoconceivable.

FIGS. 4A to 4C are diagrams illustrating representative couplingcircuits. The coupling circuit illustrated in FIG. 4A illustrates acircuit of which the input side and output side are connected by onecircuit block. It is a circuit of which the input side and output sideare connected by, for example, a distributed constant element 14 a asthe circuit block. The distributed constant element 14 a has anelectrical length of λ/4. The impedance of the distributed constantelement 14 a is approximate to the input-output impedance (for example,about 50Ω) of the filter, and higher than the impedance (for example,20Ω) of the resonant lines.

The coupling circuit illustrated in FIG. 4B is an example of a π typecoupling circuit. With the π type coupling circuit, the input side andoutput side are connected by a circuit block 141, and both ends of thecircuit block 141, and ground, are connected by circuit blocks 142 and143.

The coupling circuit illustrated in FIG. 4C is an example of a T typecoupling circuit. With the T type coupling circuit, the input side andoutput side are connected by two circuit blocks 145 and 146, and a pointbetween them, and ground, are connected by a circuit block 147. Thesecircuit blocks are realized by a distributed constant element or lumpedconstant element. The distributed constant element is, for example, amicrostrip line, and the lumped constant element is an inductor, acapacitor, or the like. Also the circuit blocks are realized with theindividual element or the combination circuit thereof.

FIGS. 4D to 4L illustrate examples of the π type coupling circuit.

The coupling circuit illustrated in FIG. 4D includes a capacitor C1connected to the signal line connecting the input terminal and outputterminal, and two inductors L1 and L2 connected between the signal lineand ground.

The coupling circuit illustrated in FIG. 4E illustrates a circuit inwhich the circuit block 141 includes a plurality of elements. Thiscoupling circuit includes two distributed constant elements 14 b and 14c, and a capacitor C11, which are connected in series to the signalline, and two inductors L11 and L12 connected between the signal lineand ground.

The coupling circuit illustrated in FIG. 4F includes a distributedconstant element 14 d connected to the signal line, and two inductorsL21 and L22 connected between the signal line and ground.

The coupling circuit illustrated in FIG. 4G illustrates a circuit inwhich each of the circuit blocks 141 to 143 includes combining aninductor and capacitor connected in parallel. This coupling circuit is acircuit in which a combination of an inductor L31 and capacitor C31connected in parallel is connected to the signal line, and a combinationof an inductor L32 and capacitor C32 connected in parallel, and acombination of an inductor L33 and capacitor C33 connected in parallel,are connected between the signal line and ground.

The coupling circuit illustrated in FIG. 4H is a circuit in which acombination of an inductor L41 and capacitor C41 connected in parallelis connected to the signal line, and two capacitors C42 and C43 areconnected between the signal line and ground.

The coupling circuit illustrated in FIG. 4I is a circuit in which adistributed constant element 14 e is connected to the signal line, and acombination of an inductor L51 and capacitor C51 connected in parallel,and a combination of an inductor L52 and capacitor C52 connected inparallel, are connected between the signal line and ground.

The coupling circuit illustrated in FIG. 4J illustrates a circuitincluding a circuit block 141 in which an inductor and capacitorconnected in parallel are combined, and furthermore, a plurality of thecombinations are connected. This coupling circuit is a circuit in whicha combination of an inductor L60 and capacitor C61 connected inparallel, and a combination of an inductor L61 and capacitor C62connected in parallel, are connected in series to the signal line, and acombination of an inductor L62 and capacitor C63 connected in parallel,and a combination of an inductor L63 and capacitor C64 connected inparallel, are connected between the signal line and ground.

The coupling circuit illustrated in FIG. 4K is a circuit in which thecapacitor C61 and the combination of the inductor L61 and capacitor C62connected in parallel are connected in series to the signal line, andthe combination of the inductor L62 and capacitor C63 connected inparallel, and the combination of the inductor L63 and capacitor C64connected in parallel, are connected between the signal line and ground.

The coupling circuit illustrated in FIG. 4L is a circuit in which aninductor L71 and a combination of an inductor L72 and capacitor C71connected in parallel are connected to the signal line, and acombination of an inductor L73 and capacitor C72 connected in parallel,and a combination of an inductor L74 and capacitor C73 connected inparallel, are connected between the signal line and ground.

FIGS. 4M and 4N are examples of the T type coupling circuit. FIG. 4Millustrates a circuit in which a combination of a capacitor C91 andinductor L91 connected in parallel, and a combination of a capacitor C92and inductor L92 connected in parallel, are connected in series to thesignal line connecting the input terminal and output terminal, and acombination of a capacitor C93 and inductor L93 connected in parallel isconnected between the signal line and ground. The coupling circuitillustrated in FIG. 4N is a circuit which includes distributed constantelements 14 f and 14 g connected in series to the signal line, and adistributed constant element 14 h connected between the signal line andground.

With a coupling circuit which, being of the π type or T type, includeslumped elements, as illustrated in FIGS. 4E, 4F, and 4I, it is possibleto connect a distributed element to one portion.

Also, the π type coupling circuit having two contact points on thesignal line connected between the input terminal and output terminal, alumped constant element or distributed constant element is connectedbetween each contact point and the ground. It is preferable that theelements connected between the contact points and ground have asymmetry. For example, it is preferable that the same element isinstalled in an a1 section and a2 section in FIG. 4D. In the event of aconfiguration wherein the same element is installed in the a1 sectionand a2 section, it is acceptable to install inductors, as illustrated inFIG. 4D or the like, it is acceptable to install capacitors, asillustrated in FIG. 4H, and it is acceptable to install a plurality oflumped constant elements, as illustrated in FIG. 4G or the like.

As illustrated in FIG. 3, by connecting the resonant lines in parallelto the signal line, and connecting the plurality of resonant lines inthe same position in the signal line, it is possible to shorten a linelength between the input line 12 a and output line 12 d, in comparisonwith a filter in which a plurality of resonators having a line length ofλ/2 are connected in series, as in a heretofore known technology, so itis possible to reduce the size of the filter in the signal linedirection.

Also, with the filter illustrated in FIG. 3, the paired resonant lines12 b and 12 c, and the paired resonant lines 12 e and 12 f, are eachdisposed in positions facing each other across the signal lineconnecting the input terminal 1 and output terminal 4. By means of thiskind of configuration, it is possible to dispose the lines in highdensity, so it is possible to further reduce the size of the filter inthe signal line direction.

With the filter illustrated in FIG. 3, the electrical length of theresonant lines 12 b, 12 c, 12 e, and 12 f is λ/4, but it is possible tomake it (λ/8)×n (λ is a resonant wavelength in the resonator, and n is apositive integer). Even in the event of using resonant lines whoseelectrical length is, for example, λ/8 (that is, n=1), it is possible toobtain the same advantages. In the event of connecting the resonantlines whose line length is λ/8, one end of each resonant line 12 b, 12c, 12 e, and 12 f is connected to ground, as illustrated in FIG. 3. Byconnecting the resonant lines 12 b, 12 c, 12 e, and 12 f whose linelength is λ/8 in this way, signals input from the input terminal 1 areinput into the resonant line 12 b via the contact point 13 a, and only asignal which meets the resonant condition of the resonant line 12 b istotally reflected from the ground end, while a signal which does notmeet the resonant condition, by being grounded, or reflected to theinput side, is attenuated. The signal totally reflected from the groundend of the resonant line 12 b has a phase difference of λ/2 from thesignals input into the contact point 13 a from the input terminal 1, andinterferes therewith. Because of this, the resonant lines 12 b and 12 cfunction in combination as one resonator. The resonated signal, afterbeing resonated again in the same way as heretofore described in theresonant lines 12 e and 12 f, via the coupling circuit 14, is outputfrom the output terminal 4 via the output line 12 g. By this means, itis possible to output a desired frequency band signal from the outputterminal 4.

A signal, among the signals input into the input terminal 1, which has awavelength which does not meet the resonant condition of a resonantline, is attenuated by being grounded, or reflected to the input endside, and is prevented from being output from the output terminal 4. Byso doing, the filter performs its function.

With the filter illustrated in FIG. 3, the pair of resonant lines on theinput side and the pair of resonant lines on the output side areconnected by the one coupling circuit 14. The number of pairs ofresonant lines included in the filter not being limited to two, it ispossible to connect three or a larger number. A connecting of a muchlarger number of paired resonant lines may be realized by connectingthem by means of a coupling circuit which couples a signal between onepair of resonant lines and another pair of resonant lines, and repeatingthis kind of connection structure. By this means, the number ofresonators included in the whole of the filter increasing, it ispossible to realize a filter with a good steepness.

FIG. 5 is a circuit diagram illustrating another configuration exampleof a filter. With the filter illustrated in FIG. 5, an input line 22 ais connected between an input terminal 1 and a contact point 23 a. Aresonant line 22 b is connected between the contact point 23 a andground. A first coupling circuit 24 is connected between the contactpoint 23 a and a contact point 23 b. A resonant line 22 c and resonantline 22 d are connected between the contact point 23 b and ground. Asecond coupling circuit 25 is connected between the contact point 23 band a contact point 23 c. A resonant line 22 e is connected between thecontact point 23 c and ground. An output line 22 f is connected betweenthe contact point 23 c and an output terminal 4.

As the first coupling circuit 24 and second coupling circuit 25, it ispossible to use the π type coupling circuit or T type coupling circuit.Also, as the π type coupling circuit or T type coupling circuit, it ispossible to employ one of the coupling circuits illustrated in FIGS. 4Ato 4N. Also, with a coupling circuit which is connected in the π type orT type, includes a lumped constant element, as illustrated in FIGS. 4E,4F, and 4I, it is possible to connect a distributed constant element toone portion.

In this way, by connecting the resonant lines 22 c and 22 d in parallelin the same portion in the signal line, it being possible to shorten theline length of the signal line connecting the input terminal 1 andoutput terminal 4, it is possible to reduce the size of the filter inthe signal line direction.

Also, with the filter illustrated in FIG. 5, a description has beengiven of an example in which the resonant lines 22 c and 22 e areconnected to ground, but it is possible to make the terminals thereofconnected to ground an open end. In this case, as it is possible toattenuate a filter passband low frequency side signal level in theresonant line 22 b, and attenuate a filter passband high frequency sidesignal level in the resonant line 22 e, it is possible to improve thesteepness of the passband characteristics of the filter.

With the filter illustrated in FIG. 5, the input side line and outputside line are connected by the first coupling circuit 24 and secondcoupling circuit 25 but, by increasing the number of coupling circuits,it is possible to connect still more paired resonant lines connectedbetween the signal line and ground.

1-3. Ground Sharing of Resonant Lines

It is acceptable that the ground ends of the resonant lines 12 b, 12 c,12 e, and 12 f illustrated in FIGS. 3 and 5 are connected one to eachindependent ground, and it is also acceptable that they are connected toan identical ground.

FIG. 6A is a plan view illustrating a specific configuration of afilter. In the filter illustrated in FIG. 6A, the circuit illustrated inFIG. 4F is employed as the coupling circuit 14. In the componentsillustrated in FIG. 6A, components identical to the componentsillustrated in FIGS. 3 and 4F are given identical reference numerals andcharacters. Also, in FIG. 6A, resonant lines 12 b′, 12 c′, 12 e′, and 12f′ connected one to each independent ground are depicted by the brokenlines, while resonant lines 12 b, 12 c, 12 e, and 12 f connected to anidentical ground are depicted by the solid lines. The line lengths ofthe resonant lines 12 b, 12 c, 12 e, 12 f, 12 b′, 12 c′, 12 e′, and 12f′ are all taken to be W1. Also, the resonant lines 12 b, 12 c, 12 e,and 12 f, and a distributed constant element 14 d, are each constructedin such a way that a plurality of capacitor electrodes straddle a signalline (a description of the capacitor electrodes will be givenhereafter). Also, the resonant lines 12 b and 12 e are connected to anidentical ground G1. Also, the resonant lines 12 c and 12 f areconnected to an identical ground G2. Also, the resonant line 12 b′ isconnected to a ground G1′. Also, the resonant line 12 c′ is connected toa ground G2′. Also, the resonant line 12 e′ is connected to a groundG3′. Also, the resonant line 12 f′ is connected to a ground G4′. Thegrounds G1′ to G4′ are physically independent of one another.

As illustrated by the broken lines in FIG. 6A, in the event that theresonant lines 12 b′ and 12 e′ are disposed so as to be perpendicular tothe signal line of which both ends are connected to input and outputterminals, and connected to the mutually independent grounds G1′ andG3′, a disposition space of a dimension W2 (W2=W1) is needed in adirection perpendicular to the signal line. In the same way, in theevent that the resonant lines 12 c′ and 12 f′ are disposed so as to beperpendicular to the signal line of which both ends are connected to theinput and output terminals, and connected to the mutually independentgrounds G2′ and G4′, a disposition space of a dimension the same as thedimension W2 is needed in the direction perpendicular to the signalline. Consequently, the size of the filter in the vertical direction(the direction perpendicular to the signal line) is W4 which isapproximately twice W2.

As opposed to this, as illustrated by the solid lines in FIG. 6A, byconnecting the resonant line 12 b and resonant line 12 e to the groundG1, and disposing them at an angle with respect to the signal line, itis sufficient that a space for disposing the resonant lines 12 b and 12e is of a dimension W3 (W3<W2) in the direction perpendicular to thesignal line. In the same way, by connecting the resonant line 12 c and12 f to the ground G2, and disposing them at an angle with respect tothe signal line, it is sufficient that a space for disposing theresonant lines 12 c and 12 f is of a dimension the same as the dimensionW3 in the direction perpendicular to the signal line. Consequently, itis possible to make a dimension W5 (W5=W3×2) of the filter in thevertical direction smaller than a dimension W4. In this way, byconnecting a plurality of resonant lines to ground at the same contactpoint, it is possible to reduce a space in which the resonant lines aredisposed.

FIG. 6B illustrates another example of a filter in which resonant linesare connected to an identical ground. The filter illustrated in FIG. 6Bdiffers from the filter illustrated in FIG. 6A in that the resonantlines 12 b, 12 c, 12 e, and 12 f are formed into an arc shape. A linelength W11 of each resonant line 12 b, 12 c, 12 e, and 12 f is the sameas the line length W1 of the resonant lines illustrated in FIG. 6A.

In this way, by forming the resonant lines 12 b, 12 c, 12 e, and 12 finto the arc shape, it is possible to make a dimension W12 of a resonantline disposition space in the direction perpendicular to the signal linesmaller than the dimension W2 illustrated in FIG. 6A. Consequently, itis possible to make a dimension W13 of the filter in the verticaldirection (the direction perpendicular to the signal line) smaller thanthe dimension W4 illustrated in FIG. 6A.

Also, it is possible to make the dimension W12 of the resonant linedisposition space in the direction perpendicular to the signal line muchsmaller than the dimension W3 illustrated in FIG. 6A. Consequently, itis possible to make the dimension W13 of the filter in the verticaldirection (the direction perpendicular to the signal line) much smallerthan the dimension W5 illustrated in FIG. 6A.

The heretofore described filter configuration which enablesminiaturization is also advantageous for a loss reduction. A loss of thefilter basically depends on a line conductor loss. By miniaturizing thefilter, it being possible to shorten the line length of the filter, itis possible to reduce a signal passing loss.

Also, by miniaturizing the filter, it being possible to increase thenumber (an available number) of filters which may be fabricated from onewafer at a time of filter manufacture, it is possible to reduce a costper element.

The filter according to the embodiment may be used as, for example, asmall GHz band frequency variable filter using an MEMS variablecapacitor.

2. Configuration of Variable Filter

With the capacitance of the resonant lines 12 b, 12 c, 12 e, and 12 fillustrated in FIG. 3, as a capacitance between them and grounds (to bedescribed hereafter) disposed in a substrate is of a fixed value, thepassband of the filter illustrated in FIG. 3 is fixed. As opposed tothis, by mounting a movable capacitor electrode (to be describedhereafter) on the resonant lines 12 b, 12 c, 12 e, and 12 f, andcoupling circuit 14, illustrated in FIG. 3, it is possible to realize avariable filter which may vary the passband. Also, by mounting a movablecapacitor electrode on the resonant lines 22 b, 22 c, 22 d, and 22 e,and coupling circuits 24 and 25 in FIG. 5, it is possible to realize avariable filter which may vary the passband. By mounting the movablecapacitor electrode on the resonant lines, it is possible to shorten theline length and, as well as it being possible to further miniaturize thefilter, it is possible to vary the passband. Also, by mounting themovable capacitor electrode on the coupling circuit, it is possible toequivalently change the electrical length of the resonator in such a wayas to provide a coupling circuit in accordance with the passband variedin the resonant lines. A description has been given of an example inwhich these variable filters have the variable capacitors, but it isalso acceptable to realize them with variable inductors. Furthermore, itis acceptable to realize the variable filters by appropriately combiningthe variable capacitors and variable inductors.

In the event that a coupling circuit including only lumped constantelements is installed in the variable filter, as illustrated in FIGS.4D, 4G, 4H, 4J, 4K, 4L, and 4M, it is sufficient to change at least onelumped constant element, among the lumped constant elements included inthe coupling circuit, to a variable element. For example, in the eventthat the coupling circuit illustrated in FIG. 4D is installed, bychanging the capacitor C1 to a variable capacitor, it is possible torealize the coupling circuit in accordance with the passband.

Also, in the event that a coupling circuit including a lumped constantelement and distributed constant element is installed in the variablefilter, as illustrated in FIGS. 4E, 4F, and 4I, by installing a movablecapacitor electrode as the lumped constant element included in thecoupling circuit, it is possible to realize the coupling circuit inaccordance with the passband.

Also, in the event that a coupling circuit including a plurality ofdistributed constant elements is installed in the filter, as illustratedin FIG. 4N, by installing a movable capacitor electrode as at least onedistributed constant element, among the distributed constant elements 14f, 14 g, and 14 h included in the coupling circuit, it is possible torealize the variable filter.

By installing the variable capacitor electrode in the resonant lines, asin the embodiment, it being possible to change the capacitance in theresonant lines, it is possible to change a signal passband in theresonant lines. By installing the resonant lines, in which the passbandis variable, in the filter in this way, it is possible to realize thevariable filter.

Hereafter, a description will be given of a specific configuration ofthe resonant lines including the movable capacitor electrode (hereafterreferred to as the variable capacitor element).

2-1. Configuration of Variable Capacitor Element

FIG. 7A is a plan view of the variable capacitor element. FIG. 7B is asectional view of a Z-Z section in FIG. 7A.

The variable capacitor element illustrated in FIGS. 7A and 7B, includinga substrate 31, a signal line 32, movable capacitor electrodes 33, driveelectrodes 35 a and 35 b, a dielectric dot 36, anchor sections 37 a and37 b, electrode pads 38, and a packaging member 39, is configured as oneportion of a filter which allows a passage of an electromagnetic wave orelectrical signal in a specified high frequency band. The packagingmember 39 seals not only a variable capacitor element section, but thewhole of the filter.

The substrate 31 is an LTCC wafer (LTCC: Low Temperature Co-firedCeramics) including multilayer internal wirings (wiring patterns 31 c).The substrate 31 is formed by mutually bonding a plurality (five in thesubstrate illustrated in FIG. 7B) of insulating layers 31 a. A via 31 bwhich includes a conductive portion in a through hole formed from oneprincipal surface to the other principal surface is formed in eachinsulating layer 31 a. Also, each wiring pattern 31 c is sandwichedbetween at least one pair of adjacent insulating layers 31 a. Also, oneportion of the wiring pattern 31 c positioned on a side of the substrate31 closest to a first surface 31 e is a ground line 31 d connected toground. The ground line 31 d faces the signal line 32 across theinsulating layer 31 a, and the ground line 31 d and signal line 32 havea gap CG2 between them. Regarding the ground line 31 d illustrated inFIG. 7B, an example has been described in which it is disposed in aposition close to the first surface but, it not being limited to this,it is also acceptable to dispose it on another layer. In this case, theground line 31 d faces the signal line 32 across a plurality of theinsulating layers 31 a. For this reason, the gap CG2 between the groundline 31 d and signal line 32 is equivalent to a thickness to which theplurality of insulating layers 31 a are stacked. Also, the wiringpatterns 31 c are connected, and the wiring patterns 31 c and electrodepads 38 are connected, by the vias 31 b. In some cases, it is acceptablethat the wiring patterns 31 c and signal line 32 are connected by thevias 31 b. Also, the insulating layers 31 a are realized by an LTCC.However, their not being limited to the LTCC, it is acceptable to formthem from another dielectric body.

The signal line 32, as illustrated in FIG. 7A, including a terminal 32 aand terminal 32 b at both ends in its longitudinal direction, is aconductor pattern in which an electrical signal passes between theterminals 32 a and 32 b. The terminals 32 a and 32 b, by being connectedto other elements on the wiring substrate, or made an open end, areelectrically connected to predetermined electrode pads 38 viapredetermined vias 31 b and wiring patterns 31 c in the wiring substrate31 (not illustrated). Also, the signal line 32, being a distributedconstant transmission line of which the impedance is, for example, 20Ω,is formed from a low resistance metal material such as, for example, Cu,Ag, Au, Al, W, or Mo. Also, the thickness of the signal line 32 is, forexample, 0.5 to 20 μm.

Both ends of each movable capacitor electrode 33 are fixed to the anchorsections 37 a and 37 b formed on the first surface 31 e of the substrate31, and a main portion thereof excluding both ends faces the signal line32 and drive electrodes 35 a and 35 b across an air gap. A thick section33 a is formed in a portion of each movable capacitor electrode 33facing the signal line 32. The thick sections 33 a and signal line 32face each other across a gap CG1. The movable capacitor electrodes 33are connected to ground via the anchor sections 37 a and 37 b, vias 31b, and wiring patterns 31 c. The movable capacitor electrodes 33, beingformed from an elastically deformable material, may be formed from, forexample, a low resistance metal such as, for example, Au, Cu, or Al. Thevariable capacitor element whose capacitance changes is realized by themovable capacitor electrodes 33 being moved to change a distance betweenthe movable capacitor electrodes 33 and signal line 32. Also, the gapCG1 between the movable capacitor electrodes 33 and signal line 32 maybe made, for example, 0.1 to 10 μm. Also, the movable capacitorelectrodes 33 and ground line 31 d are one example of a ground wiringsection in the embodiment.

The drive electrodes 35 a and 35 b, being disposed adjacent to thesignal line 32, face one portion of each movable capacitor electrode 33.The drive electrodes 35 a and 35 b generate an electrostatic attractiveforce between themselves and the movable capacitor electrodes 33,enabling the movable capacitor electrodes 33 to be displaced in adirection indicated by an arrow A. By the movable capacitor electrodes33 being displaced by the action of the drive electrodes 35 a and 35 b,a capacitance between the signal line 32 and movable capacitorelectrodes 33 changes. The drive electrodes 35 a and 35 b are formedfrom a high resistance metal thin film such as, for example, a SiCr thinfilm. Also, in order to suppress an occurrence of a pull-in phenomenon,it is preferable that a gap between the drive electrodes 35 a and 35 band movable capacitor electrodes 33 is made equal to or more than threetimes the gap CG1 between the movable capacitor electrodes 33 and signalline 32.

The dielectric dot 36, being provided on the signal line 32, is formedfrom a dielectric material such as, for example, Al2O3, SiO2, SixNy, orSiOC. The dielectric dot 36, as well as being able to prevent the signalline 32 and movable capacitor electrodes 33 from short circuiting, mayincrease a capacitance occurring in the gap CG1 between the signal line32 and movable capacitor electrodes 33. It is preferable to increase thecapacitance because it is thereby possible to ensure a wide filterfrequency variable range.

The packaging member 39 seals structures of the filter which, beingbonded to the first surface 31 e of the substrate 31, are formed on thefirst surface 31 e of the substrate 31.

In the variable capacitor element illustrated in FIGS. 7A and 7B, afirst capacitor is formed by the gap CG2 being formed between the signalline 32 and the ground line 31 d disposed in the substrate 31. Also, asecond capacitor is formed by the gap CG1 being formed between thesignal line 32 and movable capacitor electrodes 33. By forming twocapacitors in this way, it is possible to increase the capacitance.Consequently, with these capacitors, it is possible to increase thecapacitance in comparison with a microstrip line or distributed constantelement which includes only the first capacitor, as heretofore known.That is, with these capacitors, it is possible to increase thecapacitance in comparison with a microstrip line or distributed constantelement which includes no movable capacitor electrode. By increasing thecapacitance, it is possible to shorten the physical signal line lengthof a resonant line including a variable distributed constant element.Therefore, by installing this kind of variable capacitor element in thefilter, it being possible to shorten the line length of the resonantline, it is possible to miniaturize the filter.

Also, by applying a voltage to the drive electrodes 35 a and 35 b viathe electrode pads 38, vias 31 b and wiring patterns 31 c, it ispossible to generate an electrostatic attractive force between the driveelectrodes 35 a and 35 b and movable capacitor electrodes 33, andelastically displace the movable capacitor electrodes 33 in thedirection indicated by the arrow A. By displacing the movable capacitorelectrodes 33, it is possible to reduce the gap CG1 between the signalline 32 and movable capacitor electrodes 33. By reducing the gap CG1, itis possible to increase the capacitance in the second capacitor. Byincreasing the capacitance, the line length of the distributed constantelement increases equivalently or essentially, and a resonated frequencyband is shifted to a low frequency side.

Also, the drive electrodes 35 a and 35 b, as well as being divided foreach movable capacitor electrode 33, are configured so that a voltagemay be applied to each individual one. Then, by selectively applying avoltage to the divided drive electrodes 35 a and 35 b, the plurality ofmovable capacitor electrodes 33 are selectively displaced. The movablecapacitor electrodes 33 are selectively displaced, thereby enablingchanges in capacitance to differ in magnitude.

Also, as the electrostatic attractive force occurring between the driveelectrodes 35 a and 35 b and movable capacitor electrodes 33 isdiminished by decreasing the voltage applied to the drive electrodes 35a and 35 b, a displacement amount of the movable capacitor electrodes 33decreases, enabling the movable capacitor electrodes 33 to return in adirection indicated by an arrow B. By returning the movable capacitorelectrodes 33 in the direction indicated by the arrow B, the gap CG1between the signal line 32 and movable capacitor electrodes 33increases, and the capacitance in the second capacitor decreases. By thecapacitance decreasing, the electrical length of the distributedconstant element decreases equivalently or essentially.

In this way, by adjusting the voltage applied to the drive electrodes 35a and 35 b, and displacing the movable capacitor electrodes 33 in adirection approaching the signal line 32, it is possible to make thesecond capacitor a variable capacitor, and it is possible to change asignal passing frequency band in the variable filter element. It ispossible to realize the variable filter by installing this kind ofvariable capacitor element in, for example, the resonant lines 12 b, 12c, 12 e, and 12 f, and coupling circuit 14 illustrated in FIG. 3, or theresonant lines 22 b, 22 c, 22 d, and 22 e, and coupling circuit 24illustrated in FIG. 5.

Also, with a commonly known CPW signal line, a signal line (one being anexample) and ground lines (for example, two) being provided on the samesurface of a substrate, as a drive electrode for driving a movablecapacitor electrode is disposed between the signal line and the groundlines, there is a limitation on a drive electrode disposition space, andthere is a limit to increasing the area of the drive electrode. Asopposed to this, as the variable filter element using the microstripline, illustrated in FIGS. 7A and 7B has no ground line provided on asurface of the substrate the same as the surface on which the signalline is formed, it is possible to secure a large area of the driveelectrodes 35 a and 35 b on the substrate 31. By securing the large areaof the drive electrodes 35 a and 35 b, it being possible to reduce thevoltage applied to the drive electrodes 35 a and 35 b when displacingthe movable capacitor electrode 33, it is possible to secure a widemovable range of the movable capacitor electrodes 33. Also, by reducingthe drive voltage, it is possible to reduce a power consumption.

Also, it is conceivable that, by increasing the area of the driveelectrodes 35 a and 35 b, it is possible to suppress a self-actuationphenomenon due to a high frequency signal. That is, as it is possible,by increasing the area of the drive electrodes 35 a and 35 b, toincrease the electrostatic attractive force occurring between the driveelectrodes 35 a and 35 b and movable capacitor electrodes 33, it ispossible to form the movable capacitor electrodes 33 from an elasticbody with a high rigidity. Furthermore, the higher the area ratio of thedrive electrodes 35 a and 35 b and a capacitor section CAP, a coulombforce occurring between the signal line 32 and movable capacitorelectrodes 33 due to a high frequency signal passing through thecapacitor section CAP becomes negligible compared with a coulomb forceoccurring between the drive electrodes 35 a and 35 b and movablecapacitor electrodes 33 due to the drive voltage. Consequently, in theembodiment, it is conceivable that the increase in the area of the driveelectrodes 35 a and 35 b is advantageous for a suppression of theself-actuation phenomenon of a parallel plate type variable capacitor.

2-2. Method of Manufacturing Variable Filter Element

FIGS. 8A to 8G are sectional views illustrating a process ofmanufacturing the variable filter element.

Firstly, as illustrated in FIG. 8A, the electrode pads 38 are formed ona second surface 31 f of the substrate 31 including the multilayerinternal wirings. The electrode pads 38 may be formed by, for example,after forming a predetermined metal material as a film on the secondsurface 31 f of the substrate 31 by means of a sputtering method,patterning the metal film by means of a predetermined wet etching or dryetching. Alternatively, in the formation of the electrode pads 38, it ispossible to employ a nonelectrolytic plating method or electroplatingmethod. Next, the drive electrodes 35 a and 35 b are formed on the firstsurface 31 e of the substrate 31. The drive electrodes 35 a and 35 b maybe formed by, for example, after forming a predetermined metal materialas a film on the substrate 31 by means of a sputtering method,patterning the metal film by means of a predetermined wet etching or dryetching. It is also acceptable that, after the process of forming thedrive electrodes 35 a and 35 b, a process of forming an insulating filmis implemented in such a way as to cover the drive electrodes 35 a and35 b. Next, the signal line 32 and anchor sections 37 a and 37 b areformed on the first surface 31 e of the substrate 31. The signal line 32may be formed by, for example, after forming a resist pattern, which hasopenings corresponding to the signal line 32 and anchor sections 37 aand 37 b, on the substrate 31 by means of a patterning, depositing apredetermined metal material (for example, Au), and causing it to grow,in the openings by means of a plating method (a nonelectrolytic platingor electroplating).

Next, as illustrated in FIG. 8B, the dielectric dot 36 is formed on thesignal line 32. The dielectric dot 36 may be formed by, for example,after forming a predetermined dielectric film on the first surface 31 eside of the substrate 31, patterning the dielectric film.

Next, as illustrated in FIG. 8C, a sacrifice layer 40 is formed. Thesacrifice layer 40 is formed from a material which, being easy toremove, may be selectively etched.

Next, as illustrated in FIG. 8D, the movable capacitor electrode 33 isformed on the sacrifice layer 40. The movable capacitor electrode 33 maybe formed by, for example, after forming a predetermined metal materialas a film on the sacrifice layer 40 by means of a sputtering method,patterning the metal film by means of a predetermined wet etching or dryetching. Alternatively, the movable capacitor electrode 33 may be formedby employing a nonelectrolytic plating method or electroplating method

Next, as illustrated in FIG. 8E, the thick section 33 a forming oneportion of the movable capacitor electrode 33 is formed. The thicksection 33 a may be formed by, for example, after forming a resistpattern, which has an opening corresponding to the thick section 33 a,over the movable capacitor electrode 33 and sacrifice layer 40 by meansof a patterning, depositing a predetermined metal material (for example,Au), and causing it to grow, in the opening by means of a plating method(a nonelectrolytic plating or electroplating).

Next, as illustrated in FIG. 8F, the sacrifice layer 40 is removed. Bythis means, it is possible to form an air gap between the movablecapacitor electrode 33 and the signal line 32, drive electrodes 35 a and35 b, and dielectric dot 36.

Next, as illustrated in FIG. 8G, the packaging member 39 is bonded tothe first surface 31 e side of the substrate 31. As a method of bondingthe packing member 39 to the substrate 31, it is possible to propose,for example, an anodic bonding method, a direct bonding method, a roomtemperature bonding method, and a eutectic bonding method. The packagingmember 39 being one fabricated by processing an LTCC, a concavity 39 ais provided in advance in a portion corresponding to each variablefilter formation section of the substrate 31.

Next, the substrate 31 and packaging member 39 are cut into individualvariable filters.

By the above means, the variable filter is completed.

The LTCC has been used for the packaging member, but it is also possibleto use a dielectric body, such as a resin or ceramic, or a highresistance silicon.

3. Configuration of Communication Module

FIG. 9 illustrates an example of a communication module including thebandpass filter of the embodiment. As illustrated in FIG. 9, a duplexer62 includes a reception filter 62 a and transmission filter 62 b. Also,for example, receiving terminals 63 a and 63 b corresponding to balanceoutputs are connected to the reception filter 62 a. Also, thetransmission filter 62 b is connected to a transmitting terminal 65 viaa power amplifier 64. Herein, the bandpass filter of the embodiment isincluded in the reception filter 62 a and transmission filter 62 b.

When carrying out a receiving operation, the reception filter 62 aallows only a predetermined frequency band signal, among receivedsignals input via an antenna terminal 61, to pass through, and outputsit from the receiving terminals 63 a and 63 b to the exterior. Also,when carrying out a transmitting operation, the transmission filter 62 ballows only a predetermined frequency band signal, among transmissionsignals input from the transmitting terminal 65 and amplified by thepower amplifier 64, to pass through, and outputs it from the antennaterminal 61 to the exterior.

Also, FIG. 10 illustrates a communication module which includes avariable reception filter 66 a in place of the reception filter 62 a inthe communication module illustrated in FIG. 9, and a variabletransmission filter 66 b in place of the transmission filter 62 b. Thevariable reception filter 66 a and variable transmission filter 66 binclude the variable filter described in the section “2. Configurationof Variable Filter” in the present specification. With a receptionfilter and transmission filter which cannot vary the passband, in theevent of attempting to realize a multiband compatible communicationmodule which may transmit and receive a plurality of high frequencysignals in differing frequency bands, the communication module includesreception filters and transmission filters corresponding to thefrequency bands, and a switch circuit which switches between the filtersfor each of the frequency bands in which to transmit and receive thesignals, meaning that the communication module is larger in size. Asopposed to this, according to the communication module illustrated inFIG. 10, by its including one variable reception filter 66 a and onevariable transmission filter 66 b, it is possible to reduce the numberof filters, and it is possible to downsize the multiband compatiblecommunication module.

By including the passband filter of the embodiment in the receptionfilter 62 a and transmission filter 62 b of the communication module, asheretofore described, it is possible to downsize the communicationmodule. That is, with a heretofore known filter, as a configuration hasbeen employed wherein a plurality of resonant lines are connected inseries, the size in the signal line direction has been increased but, inthe embodiment, as a configuration is employed wherein the plurality ofresonant lines are connected in parallel in the same position, it ispossible to reduce the size of the filter in the signal line direction.Consequently, by mounting the miniaturized filter, it is possible todownsize the communication module. In particular, with a communicationmodule which carries out a high frequency band communication, the numberof filters becomes larger, by mounting a filter of which the size in thesignal line direction is small, as in the embodiment, it is possible todownsize the communication module. In particular, as the number offilters becomes larger with the communication module which carries outthe high frequency band communication, by mounting a filter of which thesize in the signal line direction is small, as in the embodiment, it ispossible to downsize a communication module compatible with the highfrequency band communication.

Also, as it is possible to reduce the passing loss by miniaturizing thefilter, it is possible to realize a communication module with superiorcommunication characteristics.

The configurations of the communication modules illustrated in FIGS. 9and 10 being one example, it is also possible to obtain the sameadvantages when mounting the passband filter of the embodiment in acommunication module of another form.

4. Configuration of Communication Apparatus

FIG. 11 illustrates an RF block of a portable telephone terminal as oneexample of a communication apparatus including the passband filter orcommunication module of the embodiment. Also, the communicationapparatus illustrated in FIG. 11 is illustrated as one example of aportable telephone terminal compatible with a GSM (Global System forMobile Communications) communication system and W-CDMA (Wideband CodeDivision Multiple Access) communication system. Also, the GSMcommunication system in the embodiment is compatible with a 850 MHzband, 950 MHz band, 1.8 GHz band, and 1.9 GHz band. Also, although theportable telephone terminal includes a microphone, a speaker, a liquidcrystal display, and the like, apart from the configuration illustratedin FIG. 11, as they are unnecessary in a description in the embodiment,an illustration thereof is omitted. Herein, the bandpass filter in theembodiment is included in reception filters 73 a, 77, 78, 79, and 80,and a transmission filter 73 b.

Firstly, an LSI to be operated is selected by an antenna switch circuit72 depending on whether a communication system compatible with areceived signal input via an antenna 71 is of the W-CDMA or GSM. In theevent that the received signal input is compatible with the W-CDMAcommunication system, the received signal is switched in such a way asto be output to a duplexer 73. The received signal input into theduplexer 73 is limited to a predetermined frequency band by thereception filter 73 a, and a balanced type received signal is output toan LNA 74. The LNA 74 amplifies the input received signal, and outputsit to an LSI 76. The LSI 76, based on the input received signal, carriesout a process of demodulation into a sound signal, and controls thedrive of each section in the portable telephone terminal.

Meanwhile, when transmitting signals, the LSI 76 generates transmissionsignals. The generated transmission signals are amplified by a poweramplifier 75, and input into the transmission filter 73 b. Thetransmission filter 73 b causes only a predetermined frequency bandsignal, among the input transmission signals, to pass through. Thetransmission signal output from the transmission filter 73 b is outputfrom the antenna 71 to the exterior, via the antenna switch circuit 72.

Also, in the event that the input received signal is a signal compatiblewith the GSM communication system, the antenna switch circuit 72 selectsone of the reception filters 77 to 80 in accordance with the frequencyband, and outputs the received signal. The received signal subjected toa band limitation by the selected one of the reception filters 77 to 80is input into an LSI 83. The LSI 83, based on the input received signal,carries out a process of demodulation into a sound signal, and controlsthe drive of each section in the portable telephone terminal. Meanwhile,when transmitting signals, the LSI 83 generates transmission signals.The generated transmission signals are amplified by a power amplifier 81or 82, and output from the antenna 71 to the exterior, via the antennaswitch circuit 72.

Also, FIG. 12 illustrates a communication apparatus which includes avariable reception filter 84 in place of the reception filter 73 a inthe communication apparatus illustrated in FIG. 11, and includes avariable transmission filter 85 in place of the transmission filter 73b. Also, the communication apparatus includes a variable receptionfilter 86 in place of the reception filters 77, 78, 79, and 80. Thevariable reception filters 84 and 86, and variable transmission filter85, include the variable filter described in the section “2.Configuration of Variable Filter” in the embodiment. Although notillustrated, the passbands in the variable reception filters 84 and 86,and variable transmission filter 85, are adjusted by a separatelyprovided control circuit.

As heretofore described, by installing in the communication apparatusthe filter of which the size in the signal line direction is reduced, itis possible to downsize the communication apparatus. That is, with theheretofore known filter, as a configuration has been employed whereinthe plurality of resonant lines are connected in series, the size in thesignal line direction has been increased but, in the embodiment, as aconfiguration is employed wherein the plurality of resonant lines areconnected in parallel in the same position, it is possible to reduce thesize of the filter in the signal line direction. Consequently, bymounting the miniaturized filter, it is possible to downsize thecommunication apparatus. In particular, as the number of filters becomeslarger with the communication module which carries out a high frequencyband communication, by mounting the filter of which the size in thesignal line direction is small, as in the embodiment, it is possible todownsize the communication apparatus compatible with the high frequencyband communication.

Also, by installing the variable filter, it is possible to selectivelytransmit and receive a plurality of frequency band signals using onefilter, meaning that, it being possible to reduce the number of filters,it is possible to downsize the communication apparatus. Also, as it ispossible to reduce the passing loss by miniaturizing the filter, it ispossible to realize the communication apparatus with superiorcommunication characteristics.

The communication apparatus of the embodiment is useful for a mobilecommunication apparatus of which the usable frequency band isapproximately 800 MHz to 6 GHz, in particular, a mobile communicationapparatus which carries out communication using a frequency band of 2GHz or higher.

5. Advantages of Embodiment, and Other

According to the embodiment, by connecting a resonant line functioningas a resonator between a signal line and ground, and connecting aplurality of the resonant lines in the same position in the signal line,it is possible to shorten the line length in the signal line directionin comparison with a configuration wherein a plurality of resonant linesare connected in series in the signal line direction, as in theheretofore known technology, so it is possible to reduce the size of thefilter in the signal line direction.

Also, by connecting one end of each of the resonant lines to ground, itis possible to shorten the line length of the resonant lines. In theevent that the resonant lines are connected in series to the signalline, as in the heretofore known technology, the line length of theresonant lines has been λ/2 but, by connecting the resonant lines inparallel to the signal line, and connecting one end of each of theresonant lines to ground, as in the embodiment, it is possible tototally reflect a signal which meets the resonant condition, so it ispossible to make the length of the resonant lines λ/8×n (n is a positiveinteger).

Also, by adopting a configuration wherein the resonant lines areconnected in parallel to the signal line, it is possible to mount themin high density on the substrate, so it is possible to miniaturize thefilter.

Also, by adopting a configuration wherein a plurality of the resonantlines are connected to a common ground, it is possible to dispose theresonant lines at an angle with respect to the signal line, so it ispossible to reduce the size of the filter in the vertical direction (thedirection perpendicular to the signal line).

Also, as well as connecting a plurality of the resonant lines to acommon ground, by forming the resonant lines, of each of which one endis connected to the signal line, and the other end is connected to theground, into an approximate arc shape, it is possible to reduce the sizeof the filter in the vertical direction (the direction perpendicular tothe signal line).

Also, by suspending the capacitor electrodes on the resonant lines, itis possible to increase the capacitance in the resonant lines, meaningthat, it being possible to shorten the physical line length of theresonant lines, it is possible to reduce the size of the filter in thevertical direction (the direction perpendicular to the signal line).

Also, by suspending the variable capacitor electrodes on the resonantlines, and changing the capacitance in the resonant lines by displacingthe variable capacitor electrodes, it is possible to equivalently changethe electrical length, so it is possible to realize the variable filter.By employing the variable filter as the reception filter and thetransmission filter in the communication module and communicationapparatus, there is no need to install the transmission filter andreception filter for each passband in the multiband compatiblecommunication module and communication apparatus, so it is possible todownsize the communication module and communication apparatus.

Also, by miniaturizing the filter, it being possible to increase thenumber of filter modules which may be obtained from one wafer at thetime of filter manufacture, it is possible to reduce a manufacturingcost.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions, nor does theorganization of such examples in the specification relate to a showingof the superiority and inferiority of the invention. Although theembodiments of the present inventions have been described in detail, itshould be understood that the various changes, substitutions, andalterations could be made hereto without departing from the spirit andscope of the invention.

What is claimed is:
 1. A filter comprising: a substrate; a signal lineformed on the substrate and including an input terminal and an outputterminal at either end of the signal line; a first pair of resonantlines connected between the signal line and a ground portion, whereinthe first pair of resonant lines are connected to the signal line at asame point; a second pair of resonant lines connected between the signalline and the ground portion; a coupling section provided between thefirst pair of resonant lines and the second pair of resonant lines,wherein the coupling section includes: a first circuit block connectedto the signal line and including a first terminal and a second terminal;a second circuit block connected between the first terminal of the firstcircuit block and the ground portion; and a third circuit blockconnected between the second terminal of the first circuit block and theground portion.
 2. The filter according to claim 1, wherein the couplingsection includes: a first circuit block and a second circuit blockconnected in series to the signal line; and a third circuit blockconnected between a point between the first circuit block and the secondcircuit block, and the ground portion.
 3. The filter according to claim1, wherein the plurality of resonant lines are connected to the sameground portion.
 4. The filter according to claim 1, wherein the resonantlines are formed into an arc shape.
 5. The filter according to claim 1,further comprising: a variable capacitor including a variable capacitorelectrode provided above the resonant line via an air gap and a driveelectrode for changing distance between the variable capacitor electrodeand the resonant line.
 6. The filter according to claim 1, wherein thesubstrate is a ceramic substrate including a plurality of laminatedinternal wirings.